Furfural and related compounds are useful precursors and starting materials for industrial chemicals for use as pharmaceuticals, herbicides, stabilizers, and polymers. The current furfural manufacturing process utilizes biomass such as corn cob, sugar cane bagasse, switchgrass or wood waste as a raw material feedstock for obtaining glucose, glucose oligomers, cellulose, xylose, xylose oligomers, arabinose, hemicellulose, and other C5 and C6 sugar monomers, dimers, oligomers, and polymers. The hemicellulose and cellulose are hydrolyzed under acidic conditions to their constituent sugars, such as glucose, xylose, mannose, galactose, rhamnose, and arabinose. Xylose, which is a pentose (i.e., a C5 monosaccharide) is the sugar present in the largest amount in hemicellulose. In a similar aqueous acidic environment, the C5 sugars are subsequently dehydrated and cyclized to furfural. Under similar conditions, C6 sugars can be hydrolyzed and converted in low yields to furfural.
A review by R. Karinen et al. (ChemSusChem 4 (2011), pp. 1002-1016) includes several commonly used methods of producing furfural generally as described above. All of the methods involve use of a soluble inorganic acid catalyst, such as sulfuric, phosphoric, or hydrochloric acid. These acids are difficult to separate from the reaction medium or product stream. Low yields can result from formation of undesirable byproducts. Further, their use can require increased capital costs because of associated corrosion and environmental emission issues.
As discussed by Takeshi Suzuki, et al. (Applied Catalysis A: General, 2011, Vol. 408, pp 117-124), solid acid catalysts described in the art require high reaction temperature and pressure and/or supercritical solvent to attain the selective production of furfural with a high yield and a high conversion. The solid acid catalysts are often deactivated by collection of insoluble humins or char on the catalysts.
There remains a need for a process to produce furfural at both high yield and high conversion with a solid acid catalyst, capable of operation in a batch or continuous mode, and which allows for removal of soluble humin byproduct in a batch or continuous mode.